The general litmus test for "pornography" seems to be whether it excites the
viewer or the reader. But if that's the case, then how does one distinguish between pornography and "erotica" or "obscenity"? Here's a helpful excerpt from Pornography in America: A Reference Handbook by Joseph W. Slade (ABC-Clio, 2000)

Reprinted with permission
of the publisher.

"Pornography" (or "porn") usually refers to representations designed to
arouse and give sexual pleasure to those who read, see, hear, or handle them.
Although sexual stimulation would seem to be a splendid goal, it is not always
so regarded in a society still characterized as puritanical. Opponents often
avoid dealing with the benefits of arousal in favor of attributing unflattering
motives to makers of pornography, whereas producers of pornography themselves
may cloud matters by insisting that their materials are educational rather than
deliberately stimulating. Because arguments over sexual expression mask issues
of politics, religion, gender, race, class and (above all) sexuality,
irrelevant claims and assertions are not merely typical but seemingly essential
to any discussion of pornography. At times, the confusion seems a deliberate
means of demonizing enemies, achieving political advantage, or making a
profit.

In a more general sense, the meaning of the term pornographic constantly
shifts along a vast continuum moving between two equally slippery concepts, the
erotic and the obscene. An erotic representation is usually
considered socially acceptable. Associated with upper-class sensibilities,
eroticism is primarily esthetic; erotic materials, say many critics, begin by
stimulating physical responses, then transcend them, leaving a mildly sexual
glow that one can speak of in polite company. Gloria Steinem, among others,
claims that the differences between pornographic and erotic are always obvious.
Al Goldstein, among others, maintains that such descriptions are biased by
gender, class, and factors such as personal preference: "Eroticism," says
Goldstein, "is what turns me on. Pornography is what turns you on."

At the other end of the scale are obscene representations, which are considered
to be not socially acceptable. In a legal sense, obscenity denotes criminality,
and its cultural connotation is lower-class vulgarity. In the United States,
obscene material can be prosecuted because of its nastiness, its demeaning
"prurience," or its sheer inhumanness. By contrast, pornography is entirely
legal. Sexual expression is free to arouse, but only within limits, and those
limits, which are set by concepts of obscenity, erode only over time. "I know
it when I see it," Supreme Court Justice Potter Stewart once said of obscenity.
Because Stewart was virtually blind, his comment illustrates the difficulty of
deciding what is obscene rather than pornographic. Stewart's remark is also as
close as the American judicial system has come to a definitive statement on the
issue. If a representation transgresses against sexual norms (that themselves
change), courts may judge it obscene; if it does not push against boundaries,
however, it may not arouse. In contrast, when a representation once called
obscene becomes so widespread that taboos against it weaken, it moves first
into the category of the pornographic, then of the erotic. From the domain of
the erotic, the representation (a public kiss, for example) can pass into the
realm of the commonplace.

The problem, of course, is that not everyone uses the same measurements. Some
Americans believe that sex is a necessary evil, sanctioned only by marriage for
purposes of reproduction, and condemn sexual representations under any
circumstances. At the other extreme, those who concede that sex can and should
be recreational may nonetheless find some types of representation disturbing. A
reader comfortable with a sexual scene in a novel, for example, may be repelled
by the same scene in a movie or on stage. Others attempt to distinguish between
degrees of explicitness -- how much flesh is visible, say, or how vulgar a spoken
word, or what kind of sexual act is depicted.

For most Americans, pornography means peep shows, striptease, live sex acts,
hardcore videos, adult cable programming, sexual aids and devices, explicit
telephone and computer messages, adult magazines, and raunchy fiction.
Conservatives might add prime-time television programming, soap operas, Music
Television (MTV) and rock music, romance novels, fashion magazines, and all
R-rated movies. Conflating sexuality and violence leads some critics to think
of sexual representations as inherently aggressive. Others, noticing that most
sexual representations contain no violence, condemn only those examples that
mix the two. As Walter Kendrick has pointed out, pornography is not a thing but
an argument.

To avoid contentiousness, some theorists prefer a neutral term such as sexual
materials over the charged word pornography. In any case, only a few
things seem clear. First, what seems pornographic to one person will not
necessarily seem so to another. Second, pornography is not monolithic:
representation occurs in many media, and it adopts many forms and genres.
Third, no group, gendered or otherwise, has a monopoly on sexual expression or
representation. Fourth, our social, esthetic, political, legal, and economic
attitudes toward pornography both affect and draw on complex responses to
gender and sexuality. Fifth, pornography, an attempt at communication, conveys
a host of messages, many of them contradictory. Some of those messages, in
fact, are ancient.